Diffusion barriers on Cu surfaces and near steps

Abstract

This paper reports a molecular statics study of Cu surface diffusion barriers, particularly the facet–facet and step–facet barriers. The study focuses on two high-symmetry surfaces or facets, Cu{111} and Cu{100}. Our results show that these two barriers are distinct from conventional step barriers and are independent of facet size once it is beyond three atomic layers. Usually, the facet–facet barrier is substantially larger than diffusion barriers on flat surfaces or down monolayer steps, and the step–facet barrier is substantially larger than diffusion barriers along or across monolayer steps. Exceptions do exist. When two Cu{100} facets are involved, the two barriers decrease as the size of the ending facet increases from one layer to two layers, and then increase from two to three (or more) layers. As a result of the large facet–facet and step–facet barriers, surfaces of Cu thin films are of the order of 100 nm. The small facet–facet and step–facet barriers between two Cu{100} facets, when the ending facet is two to three layers, make it difficult to form another Cu{100} facet near one Cu{100} facet. For the same reason, nanowires along 100/{100} on the Cu{100} are unlikely, while nanowires along 110/{111} are feasible.